The risk of thermal fatigue, and the resulting pressure boundary failures, have resulted in a significant amount of research in the Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR) communities in the past two decades. Thermal stresses arising from thermal stratification phenomena are typically not taken into account in design calculations. Because various forms of thermal fatigue events occurred in safety-related piping, non-design basis thermal fatigue has been the subject of a number of U.S. NRC bulletins that led to a significant amount of research by the nuclear industry. This paper discusses the relevance of known thermal fatigue mechanisms to the CANDU® design (a pressurized heavy water reactor), and thermal fatigue operational experience in CANDU plants. These mechanisms are various forms of thermal stratification phenomena, turbulence penetration and large-scale temperature oscillations in mixing fluid streams.

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